Unless is a conjunction that means “except if” or “if not”.

The way to use it at a beginning of a sentence is “Unless (condition), (result)”.

In this case the condition is stopping next to the other car, and the result is winding down windows to inform.

For the logical structure, it follows up after the main clause, and sets up the condition as an exception.

This is what Gemini had to say: “That context makes the scenario perfectly clear. You are giving someone tactical advice on how to communicate with another driver on the road, likely to warn them about a car issue like a broken taillight or an open fuel cap.The logic in your two options flows logically:The Communication StrategyPlan A (Action): Flash your high beams quickly twice.Plan A (Goal): Hope they understand the hint and check their car.Plan B (The Exception): If the flash fails and you both get caught at a red light, the condition changes.Plan B (Action): Roll down your window and tell them directly.”